1. Field of the Invention
The invention relates to an alternating-current motor control apparatus, and more particularly, an alternating-current motor control apparatus that has a rectangular wave voltage control mode that drives an alternating-current motor by applying rectangular wave voltage to it.
2. Description of the Related Art
Typically, a structure is used that converts direct-current voltage (DC voltage) using an inverter, and then uses this DC voltage to drive an alternating-current motor (AC motor). With this kind of structure, the motor current is typically controlled according to pulse width modulation (PWM) based on vector control in order to drive the motor efficiently.
Also, a structure is also known that controls an AC motor by switching between rectangular wave voltage control that drives the AC motor by applying rectangular wave voltage to it and PWM control in order to improve motor output. For example, Japanese Patent Application Publication No. 2007-159368 (JP-A-2007-159368) describes AC motor control for preventing a fluctuation in output torque when switching the control method between rectangular wave voltage control and PWM control.
With the motor control method described in JP-A-2007-159368, torque feedback control according to phase control of the rectangular wave voltage pulse based on the difference between the actual torque value and a torque command value is executed when rectangular wave voltage control is selected. In this torque feedback control, an electronic control unit (ECU) that forms a rectangular wave voltage control portion samples the motor current detected by a current sensor at predetermined intervals and estimates the actual torque value using the sampled motor current detected value. That is, in the rectangular wave voltage control, the sampling accuracy of the motor current is directly linked to the estimation accuracy of the actual torque value.
Here, with sine wave PWM control which is used for typical PWM control, the line voltage applied to the AC motor is a sine wave because the amplitude of the sinusoidal voltage command is restricted to equal to or less than the amplitude of the carrier wave (which is representatively a triangular wave). Therefore, the fundamental wave component of the voltage applied to the AC motor is unable to be increased more than approximately 0.61 times the DC link voltage of the inverter.
In contrast, with rectangular wave voltage control, the modulation factor (i.e., the ratio of the fundamental wave component (i.e., the effective value) of the motor applied voltage (i.e., the line voltage) to the DC link voltage of the inverter) can be increased up to 0.78 by applying one rectangular wave pulse, in which the ratio of the high level time to the ON time of the upper arm element of each phase and the ratio of the low level time to the ON time of the lower arm element of each phase is 1:1, to the AC motor. However, the motor applied voltage includes a harmonic component and is therefore distorted instead of a sinusoidal. As a result, the motor current flowing through the AC motor includes a distortion component instead of a sine wave. Therefore, in order to ensure the estimation accuracy of the actual torque value, it is necessary to execute the control together with a filtering process for removing the distortion component from the sampled motor current.
However, incorporating this kind of filtering process into the actual torque value estimating process results in a delay due to the time constant of the filter, so control responsiveness may decrease. In particular, when the rotation speed of the AC motor is low, the control cycle of the torque feedback control increases, and as a result, the delay effect from the filtering process becomes pronounced. On the other hand, when the time constant of the filter is reduced to eliminate the day, the effect from the distortion component in the motor current is unable to be suppressed. As a result, torque fluctuation may occur in the AC motor due to a decrease in the estimation accuracy of the actual torque value.
Incidentally, a control structure in which the motor current is sampled and the filtering process is performed in parallel is effective to avoid this kind of problem. However, this kind of control structure severely increases the load on the ECU, which may lead to an increase in the cost of the apparatus.